IAEA progress report (9 months)

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i. IAEA Research Contract No: 17810

ii. Title of Project: »MONITORING OF MATERIAL DEGRADATION DURING LONG TERM STORAGE OF SPENT FUEL«

iii. Institute where research is being carried out: »ZAG – Slovenian National Building and Civil Engineering Institute«

iv. Chief Scientific Investigator: Andraž Legat

v. Time period covered: May 2013 – Jan. 2014 (9 months) ????

Detection of stress corrosion cracking (SCC) in stainless steel

In the reported period the majority of the research activities were devoted to enhancement of skills and knowledge required to measure and to detect SCC events primarily in the autoclave environment. The following topics were investigated for the autoclave environment:

· evaluation of acoustic emission measurement for SCC detection

· selection of reference and counter electrodes and their positioning during EC noise measurement + the effect of surface scratching

· sample preparation and cleaning

· crack growth

General for SCC measurements in the autoclave:

Stress corrosion cracking of AISI304 tensile samples (solution annealed 1h @ 1050°C, then sensitized 24h @ 650°C) was initiated during the slow strain rate test (SSRT) with relative elongation rate d/dt=2×10-6/s. The environment was demineralized water with initial R.T. conductivity in the 1.0-1.5 S/cm range while the conductivity at the end of the experiment increased to the 5-8 S/cm range. Autoclave operating conditions were 288°C which corresponds to 73 bar of water partial pressure, in certain measurement campaigns the total pressure was raised by adding approximately 15 bar of Argon gas. The tensile sample was cut from 3mm-thick AISI304 rolled plate and heat treated afterwards. The neck length is 25mm. Entire tensile sample was submerged in hot water during the SSRT.

Electrochemical potential and current noise were measured in a freely corroding system consisting of three electrodes: counter (CE), reference (RE) and working electrode (WE), fig F1. Only the working electrode (tensile sample) was exposed to tensile load. Current noise was measured by zero resistance ammeter and voltage noise by high-impedance voltmeter. Both units are part of the electrochemical noise measurement device HRU/ZRA-FG-B-2M (IPS, Germany) and Gill AC (ACM Instr., USA). Both of those instruments were used in the current study.

Figure 1: Scheme of the electrochemical noise measurement circuit.

Evaluation of acoustic emission measurement for SCC detection:

The only option for mounting the piezoelectric AE sensor is on the pull-rod, since the sensors cannot be attached to the tensile sample itself. Through several dedicated experiments it has been shown that no useful AE signals from the sample can be detected. Two main reasons for this have been identified:

· weak coupling between AE sensor and tensile sample due to sample holder consisting of several pieces (including the ceramic isolators), significant length of the pull-rod and additional attenuation caused by the pull-rod seals.

· high background noise caused by the stepper motor used for pull-rod movement during SSRT. This, almost continuous, noise is present (though at lower amplitudes) even when the stepper motor is at rest and the noise can be eliminated only by disconnecting the stepper motor power supply.

Dedicated experiments involved a special, larger tensile sample (AISI304 steel) to which one AE sensor was attached, while the second AE sensor was attached to the usual location on the pull-rod. The tensile test with constant elongation rate of 1.5m/s was carried out with open autoclave in air at R.T. After the sample fracture the AE signals from both sensors were compared.

Selection of reference and counter electrodes and their positioning during EC noise measurement

During multiple measurement campaigns we realized that Ag/AgCl reference electrode supplied by the autoclave producer (Cormet) might not be the best option, because of intrinsic noise and large distance between the tensile sample neck and reference electrode porous ceramic plug. For this reason we applied the stainless steel wire as a pseudo-electrode and positioned it no more than few mm from the tensile sample neck in the form of C ring. The resulting recorded ECN current and voltage are significantly more representative for the sample undergoing SCC, fig 2. It is evident that cracks form almost until the maximum stress. The final fracture peak and one peak before (fig. 3) are nicely correlated to drop in force, moreover, the direction and shape of the voltage and current peaks are in agreement with the creation of fresh surface. Thanks to scratching device, the effect of WE and CE scratching (removal of passive layer) on the ECN have been also investigated, fig. 4. This confirms that electrochemical noise measurement can be used also in poorly conductive media (pure water) and is highly sensitive method.

SEM analysis of the fractured sample revealed that SCC started at the surface as transgranular (TG) SCC and then proceeded mostly as intergranular (IG) SCC toward the bulk. The rest (majority) of the fracture area is of ductile nature and fractured suddenly when the stress increased above the tensile strength.

Figure 2: ECN current and voltage, and force vs. time during the SSRT. The sample did not fracture suddenly but rather continuously.

Figure 3: Details of ECN voltage and current peak due to sample fracture and relaxation that caused fresh surfaces.

Figure 4: ECN current and voltage temporal evolution after the scratching of working electrode and subsequent scratching of counter electrode.

Figure 5: SEM images of the fracture plane with labels denoting the nature of the fracture.

Sample preparation and cleaning

Tensile samples were intentionally sensitized in order to facilitate SCC. This heat treatment was done in partially protective atmosphere which still led to oxidation of samples. Initially we used chemical cleaning (descaling) according to ASTM A380 standard. This comprised of treatment in solution of nitric acid, in solution of nitric + hydrofluoric acid and final passivation in solution of nitric acid. After several SSRT measurement campaigns it was found that such cleaning initiates intergranular corrosion which was the major reason for sample failure, hence the SCC most likely did not occur at all. For this reason all further studies were done on mechanically cleaned (brushed) samples. The effect of the chemical cleaning was additionally investigated on the tensile sample prior and after the SSRT measurement in the autoclave. This clearly confirmed, fig. 6, that chemical cleaning alone causes intergranular corrosion with crack depths around 0.1mm (1-2 grain boundaries deep). The SEM images of the mechanically cleaned sample after the SSRT measurement reveal only cracks due to SCC, fig. 7.

Figure 6: SEM and metalographic image of chemically cleaned sample prior (left column) and after (right column) the SSRT measurement in the autoclave.

Figure 7: SEM image of the mechanically cleaned sample after the SSRT measurement.

Monitoring of the cracks growth

Slow strain rate test of one selected sample was terminated and the sample was removed from the autoclave for non-destructive imaging with X-ray microtomograph. Afterwards the SSRT on this sample continued till the fracture. Then the imaging was done again. These two 3D images enabled the investigation of the cracks growth. In this particular study the increase of crack dimension was relatively small, since “before” fracture and “after” fracture image were both taken at relatively high and similar strain (29% and 38% respectively), fig. 8.

Microtomograph images of selected section planes were filtered using a threshold filter to obtain a binary image which allowed for dimensional and area measurement of cracks, fig. 9-11.

Figure 8: Stress vs. time (strain) curve for the sample used for crack-growth study.

Figure 9: 3D microtomograph image of the outer surface of the tensile sample. Arrow indicates the same crack to guide the eye. (?? te slike morda raje ne, ker je kemično čiščen vzorec – ogromno razpok)

Figure 10: Comparison of subsurface section plane; left before, right after, with area and depth measurements and increase in %.

Figure 11: Section plane 1.1 mm below surface. Top left: grayscale image of “before” and “after”, top right: threshold-filtered image of “before” and “after”. Bottom: the selected crack (between the dotted line), before and after. Purple contour denotes the crack boundary as defined by the threshold filter.

Detection of SCC in AISI321 at constant load in NaCl & H2SO4 solution at 70°C [1]

Test samples made from the as a received AISI 321 plate of the thickness 1 mm were subjected to static load near yield point to provoke SCC. Experiments were carried out in 0.5M NaCl + 0.5M H2SO4 (pH was 2) at the elevated temperature of 70 oC. Experimental setup included electrochemical noise (EN) monitoring in freely corrosion system, acoustic emission technique (AE), measurement of tensile test sample elongation and simultaneous digital imaging of the measured sample surface by CCD camera. For the electrochemical noise measurements working electrode was in the shape of tensile test sample and subjected to static tensile load, counter and reference electrode were made from the same material and were not exposed to the load, fig. 12. Both AE sensors were fixed directly to the tensile sample, one to the top and one to the bottom. After the end of test, optical (scanning electron microscopy, metallography) techniques were used to characterize the nature of SCC processes obtained during the experiments. Besides the mentioned experiments, also basic electrochemical characterization of selected material in selected environment was determined.

Monitored parameters during one of the representative measurement of can be seen in fig. 13. Significant change of trend of ECN current and voltage signal approximately 4 hours after the start of experiment corresponds very well with beginning of tensile sample elongation increasing. Comparison of AE intensity histogram with other monitored parameters did not reveal any correlation. However, detailed examination of captured images has shown that the highest AE intensity corresponds well to the intensity of bubbles formation on the surface of tensile test sample. After that period, taking place approximately from 6 to 10 h after the beginning of the experiment, bubbles formation slowly decreased, and cracks starts to initiate and propagate on the surface (also clearly observed by digital imaging system).

SEM and metallographic analysis carried out on cracked samples have shown number of cracks appeared on the tensile test samples, fig. 14. Majority of crack paths was transgranular. Interior walls of cracks were severely corroded because of aggressive environment. Since no significant sudden (sharp and high) transients in electrochemical voltage and current noise were found it can be assumed that the uniform dissolution of material is a dominant corrosion process taking place in this corrosion system. Once the crack is formed, newly exposed area of cracks cannot be sufficiently passivated and they subsequently corrode. Corrosion inside of cracks blunts otherwise their sharp tip, due to this reason cracks are short.

Figure 12: Experimental scheme for SCC measurement at elevated temperature and constant tensile load.

Figure 13: Voltage (U) and current (I) noise, velocity of tensile test sample elongation, elongation and AR intesity of sensor 1, 2 and total intensity of both sensors during the total duration of SCC test

Figure 14: Surface of tensile test sample near the fracture spot.

References:

[1]  BAJT LEBAN, Mirjam, ZAJEC, Bojan, KOSEC, Tadeja, LEGAT, Andraž. Monitoring the behaviour of AISI 321 stainless steel under constant tensile loading. In conference proceeding: Risk Conference 2013 : Washington, Wednesday, June 19, 2013. Washington; NACE, 2013

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